1. Field of the Disclosure
The present disclosure relates to a power amplifier for use in a digital portable communication terminal, and in particular to a power amplification apparatus, an OFDM modulation apparatus, a wireless transmission apparatus, and a distortion reduction method for a power amplification apparatus that allow reduction of distortion produced by a power amplifier.
2. Description of the Related Art
In the field of high-speed mobile communication, a new scheme called LTE (Long Term Evolution) is currently being developed and put into practical use. For modulation in transmission (uplink from a terminal), the LTE uses diffusive orthogonal frequency-domain multi-signal modulation (SC-FDMA: Single Carrier Frequency Division Multiple Access), which is a variation of OFDM (Orthogonal Frequency Division Multiplexing). Transmission apparatuses that utilize the SC-FDMA are disclosed in Japanese Unexamined Patent Application Publication No. 2009-239539 and Japanese Unexamined Patent Application Publication No. 2009-239723.
In the SC-FDMA, the peak average power ratio (PAPR) of a signal input to a power amplifier is about 5.5 dB, which is higher by 2 dB or more than that in W-CDMA (Wide-band Code Division Multiple Access), which is a scheme according to the related art. Thus, more severe restrictions are imposed on the power amplifier. In order to avoid this issue, it is conceivable to decrease desired power. However, this approach naturally decreases the communication speed, which contradicts to an original purpose.
As an example of techniques associated with the improvement of the PAPR according to the related art, Japanese Unexamined Patent Application Publication No.2007-208729 proposes an OFDM communication device that allows reduction of peak power of an OFDM time waveform without wasting an FDD allocated band. In this technique, a peak portion of a signal input to a power amplifier is detected. Based on a detected distortion factor, a shaped waveform for reducing the distortion factor is generated. Modulation results obtained by modulating the shaped waveform are synthesized with a modulation signal obtained by modulating transmission data in a band other than a band allocated to the transmission data to generate a modulation signal from which the distortion factor has been reduced.
As another example of techniques according to the related art, Japanese Unexamined Patent Application Publication No. 2005-86440 discloses “METHOD AND DEVICE FOR TRANSMITTING OFDM SIGNAL”, in which distortion at an output of a power amplifier is detected and superimposed on an OFDM signal to generate a pre-distortion signal and linearize the output of the power amplifier.